In the process of manufacturing semiconductor integrated circuits, wafers are usually held in place by wafer chucks of the electrostatic attraction or Johnsen-Rahbek effect type, that is, so-called electrostatic chucks. The electrostatic chuck includes an insulating layer, and organic resins such as polyimides, ceramics such as alumina and aluminum nitride, and rubber elastomers such as silicone rubber have been used as the insulating layer.
In the plasma etching step, it is essential to inhibit a wafer from being heated by the plasma and maintain a uniform and constant temperature distribution over the wafer. To this end, the electrostatic chuck is provided on the back side with a cooling mechanism such as a chiller. Given a uniform and constant temperature distribution, a high selectivity is established between the masking material and the underlying substrate to be etched and an anisotropic feature is readily etchable. These features ensure high accuracy etching.
The electrostatic chuck using a ceramic insulating layer is fully durable against a plasma gas and highly heat conductive. The ceramic insulating layer, however, is so hard that its contact with a wafer is not fully intimate because the wafer has an irregular surface. This results in an increased thermal contact resistance and an insufficient cooling capacity. To promote heat transfer, a gas cooling system wherein an inert gas such as helium flows between the wafer and the insulating layer is employed. This system, however, requires a fine working step of forming inert gas flow channels in the insulating layer surface and a feed source of the inert gas, and is disadvantageous in that the ESC structure becomes complex and the ESC manufacture cost is increased. Physical contact and rub between the ceramic insulating layer and a wafer generate microscopic particles which deposit on the back side of the wafer so that they are carried over to the subsequent process, causing the problem of defects in the wafer microprocessing.
The electrostatic chuck using a polyimide insulating layer is easy to manufacture and inexpensive, but less durable against a plasma gas. Undesirably it has a low heat conductivity and a high hardness which leads to an increased thermal contact resistance and an insufficient cooling capacity. Like the ceramic insulator, it also suffers from the problem of particle generation.
JP-A 59-64245 discloses an electrostatic chuck comprising a metal plate, a first insulating film on the metal plate comprising a heat dissipating silicone prepreg obtained by impregnating glass cloth with silicone rubber, a copper pattern formed on the first insulating film as an electrode, and a second insulating film of silicone rubber on the copper pattern. Since the silicone rubber or elastomer is used in the insulating layer, this ESC has a relatively low thermal contact resistance and a good cooling capacity. The use of this ESC is effective in maintaining the wafer temperature uniform. Since the silicone rubber is soft, few particles are generated by contact with the wafer. The number of particles generated is reduced as compared with the ceramic and polyimide layers. However, a heat conductive filler such as alumina, zinc oxide or boron nitride which has been added to the silicone rubber for imparting heat conductivity thereto will escape and fall out, becoming particles. While an ESC having minimal particle generation is needed to meet the current demand of reducing wafer processing line width toward higher integration, this ESC is regarded short in performance.
JP-A 2-27748 discloses an electrostatic chuck comprising a first insulating layer of polyimide film, an electrode layer of copper foil, and a second insulating layer of polyimide film laminated on a metal substrate in sequence using an adhesive, a workpiece to be held being rested on the second insulating layer, wherein a contact improving layer is formed on the second insulating layer for improving the thermal contact with the workpiece. This ESC, however, has the risk that the contact improving layer itself can be transferred to the wafer or the heat conductive filler in the contact improving layer can fall out, causing particle generation.
JP-A 10-335439 discloses an electrostatic chuck comprising a metal substrate, a first insulating layer of silicone rubber thereon, an electroconductive pattern on the first insulating layer, and a second insulating layer of silicone rubber on the conductive pattern having an embossed surface, wherein the amount of particles depositing on a wafer is reduced. This ESC, however, has the risk of particle generation because the heat conductive filler in the silicone rubber can fall out.
JP-A 11-233604 discloses an electrostatic chuck having a suction surface on which a protective film of silicone resin, epoxy resin or acrylic resin is formed, the chuck having improved resistance to abrasion and damage. Where this protective film is hard, the wafer cooling capacity becomes low and rubs with a wafer can generate particles. The filler other than silica in the protective film can fall out, causing particle generation.
JP-A 2004-253718 discloses an electrostatic chuck comprising a ceramic substrate and an electrode embedded therein, the chuck having a suction surface on which a tacky/anti-slip layer of silicone resin, polyimide resin or the like is formed. This ESC allows for easy alignment of a workpiece to be suction held and causes no damages to the workpiece. Likewise, this ESC has the risk of particle generation from the tacky/anti-slip layer.